Most cameras, including digital cameras, comprise an autofocus feature with which objects in a viewed scene can be automatically focused by the camera. The autofocus functionality can either be continuous, wherein the camera continually adjusts the camera focus as the viewed scene changes, or single, wherein autofocusing only occurs when a user depresses (e.g., halfway depresses) a shutter button.
Irrespective of the autofocus mode that is used, focusing is typically achieved by analyzing the viewed scene with a focusing algorithm. In particular, discrete portions of the viewed scene are analyzed independently and values are assigned to each as to the degree of focus that is observed. These portions may comprise portions of the entire scene, or only a part of it (e.g., the center of the scene). The values are assigned to the various analyzed portions by evaluating the perceived sharpness of objects in each portion. After the analysis has been conducted and values assigned, the lens system is manipulated to alter the focus, and the analysis is conducted again to generate new values for the various portions. The new values for the portions are then compared to those previously assigned to the respective portions to determine whether the focus improved or got worse. This process continues until the optimum focus has been determined.
The autofocus method described above works well in most conditions. Sometimes, however, unintended results can occur. For example, in situations in which the subject (e.g., a person) is in the foreground of a viewed scene, but higher contrast objects are in the background, the camera may, contrary to the user's intent, focus on the background instead of the subject. To cite another example, if the subject is to the side within a viewed scene, the background (which occupies the center of the framed scene) may be used as the object of interest by the autofocus system. Therefore, if the user's friend stands before a mountain range but is not in the center of the composed shot, it is likely that the mountain range, and not the friend, will be in focus.
Although such problems can typically be avoided by first focusing only on the subject, locking the focus (e.g., by pressing the shutter button halfway), and then composing the picture before capturing an image, most casual camera users are not that savvy. Therefore, many users capture images in which objects are out-of-focus.
One benefit of digital cameras is that they allow the user to immediately view a captured shot. Despite this capability, the user is not likely to detect an out-of-focus condition in that the displays of most cameras are too small, and their resolutions are too low, for the user to readily identify this condition. The situation is even worse when the display is used to compose the shot. In that live view images shown in the display while a picture is being composed are typically very low resolution images (to enable images to be shown real time), it is very difficult for the user to tell whether the subject is or is not in focus.